In recent years, Dense Wavelength-Division Multiplexing (DWDM) and Erbium-Doped Optical Fiber Amplifier (EDFA) have become mature technologies, the large-capacity long-distance communication systems including fibers and EDFAs are increasing, and international submarine communication systems are increasing. Therefore, faults of submarine lines should be located quickly and accurately, so that maintainers may remove the faults quickly, which can reduce operation expenditure of the submarine communication system.
Generally, a submarine system includes two Submarine Line Terminal Equipment (SLTEs) and multiple RPTs. Each RPT is configured to amplify an optical signal which is previously attenuated in a link. In the RPT, the EDFAs of each pair of fibers share a pump laser, as shown in FIG. 1. In the prior art, a Coherent Optical Time Domain Reflector (COTDR) technology is most widely used to locate faults of submarine links. Similar to the principles of an existing Optical Time Domain Reflector (OTDR), the COTDR uses Rayleigh scattering and Fresnel reflection to represent fiber characteristics, but the COTDR differs from the principles of the OTDR in that the COTDR uses coherent detection on a receiver to improve the signal-to-noise ratio of a received signal.
FIG. 2 is a schematic brief diagram of hardware which uses a COTDR technology to locate faults of submarine cables. A controller in a terrestrial detection device controls a probe light source to output probe light. The probe light is divided by a 3 dB coupler into two parts. One part is local oscillation light for coherent detection, and the other part is shifted and modulated by an acoustooptic modulator into pulse light. The pulse light and a service signal (namely, main signal in FIG. 2) are coupled together by a wavelength division multiplexer into the fiber as the probe light. The probe light is reflected back to an input side once the probe light pulse runs across fiber joints, break points, break planes, endpoints or other defective points of the fiber, and the reflected light is captured by a probe on the input side. Besides, non-uniform particles smaller than the wavelength in the fiber material lead to Rayleigh scattering. A minor part of the scattered light is transmitted inversely to the input side along the fiber, but the light cannot be reflected or scattered back along the original route. Therefore, a 10 dB beam splitter is added after every EDFA in the RPT (the RPT is the same as the RPT in FIG. 1), so that the light may be reflected and scattered back along a reverse path of the fiber. A wavelength division multiplexer in the terrestrial detection device separates the reflected light and the scattered light from the main signal. The reflected light and the scattered light are filtered by an optical filter, and are incident together with the local oscillation light onto the surface of the probe by a coupler. On the surface of the probe, the light is received coherently. The probe converts the optical signal into an electric signal. The controller processes the electric signal to obtain a loss characteristic curve of the fiber. The loss characteristic curve of the fiber is displayed in a monitor.
When both the transmission line and the EDFA are normal, because the backward scattered light of the probe light is amplified by the EDFA persistently, the backward scattered light received by the COTDR is a series of sawtooth waves. As shown in FIG. 3, the peak value of each sawtooth represents signal strength output by each EDFA after the backward scattered light passes through the EDFA, and the hypotenuse of the sawtooth means that the backward scattered optical power attenuates with the increase of the transmission distance. If the link is cut, because the Fresnel reflected light is much stronger than the Rayleigh scattered light, the strength of the optical signals which are on the curve and detected by the COTDR attenuates quickly. For example, location A in the figure is a fiber cut.
The backward scattered light performs Amplifier Spontaneous Emission (ASE) whenever it passes through an EDFA, and may pass through multiple EDFAs when it arrives at the probe. Therefore, much ASE noise is accumulated along the link. To obtain the accurate location of the fiber cut detected through the curve in FIG. 3, the probe light needs to emit many light pulses, and many averaging operations need to be performed on the receiver to improve the signal-to-noise ratio of the signals. For example, if a single span of a 12000 km submarine link is 100 km, the link requires 120 EDFAs, and the number of amplifiers and the accumulated ASE noise spectrum density are calculated through the following formula (1):DASEN=·[2·nsp·(G−1)·h·ν]  (1)
In the foregoing formula, DASE is the spectrum density of the accumulated ASE noise, N is the number of EDFAs, nsp is the spontaneous emission factor of the EDFA, G is the gain of the EDFA, h is a Planck constant, and ν is an optical central frequency. According to general EDFA parameters, the accumulated noise of the 12000 km link may be calculated. In order to detect the 12000 km link by using the COTDR, at least 216 averaging operations need to be performed. One averaging operation requires the pulse to finish a round trip of 12000 km. According to the propagation speed of light in the fiber, the time consumed by the 216 averaging operations may be calculated, which is not less than 2 hours.
In the process of developing and practicing the prior art, the inventor of the present invention finds that in the method for locating a fault of a submarine cable system in the prior art, the probe light needs to passes through multiple EDFAs when traveling back to the COTDR, and ASE noise is accumulated; consequently, multiple averaging operations need to be performed; in each averaging operation, the probe light pulse travels from the point of emitting the probe pulse to the point of the fiber cut, and then travels back from the point of the fiber cut to the point of emitting the probe pulse. Therefore, it takes too much time to locate the fault of the submarine line in the prior art, and the fault cannot be located in time.